The STEM is being used for mass measurement and the possible localization of specific groups by Au-labeling (carbohydrate or SH groups) in the study of the molluscan hemocyanins (Hc). The simplest of these decameric cylindrical macromolecules is ten times the size of the arthropodan hexameric Hc. The Hcs of marine gastropods consist of multiple decameric cylinders assembled into di-decamers, and of particular interest, tri-decamers and higher arrays. The chiton decamer preparation gave mass measurements of about 4.3 million in good agreement with light scattering results. The five-fold symmetry of the molecule was clearly visible. For a di-decamer, a value of 8.5 MD was obtained. B. Contrarium gave molecular weights which were integer multiples of 4.3 MD for integers ranging from 2-7 and the vertical oligomers showed the linearity of mass measurement with thickness. The automass program was useful in processing large quantities of data. Many other hemocyanins have been compared to try to discover what determines the preferred number of decamers for a particular organism. Carbonic anhydrase is the enzyme used by oyster in shell formation. This has been purified and shown to be composed of 35kD subunits. Unlike the vertebrate complex which functions as a 30kD monomer, the snail enzyme is a complex of roughly 2MD. STEM mass measurements are being used to determine the number and spatial arrangement of subunits.